Axion perturbation spectra in string cosmologies

TL;DR

This paper analyzes the spectra of axion perturbations in string cosmology models, showing that they can be nearly scale-invariant and discussing their implications for cosmic microwave background observations.

Contribution

It provides a calculation of axion perturbation spectra in a type IIB string model within a pre big bang scenario, highlighting their potential scale-invariance.

Findings

Axion spectra can be nearly scale-invariant on large scales.

Some axion fields may produce red or scale-invariant spectra.

The spectra relate to the symmetries of low energy string theory.

Abstract

We discuss the semi-classical perturbation spectra produced in the massless fields of the low energy string action in a pre big bang type scenario. Axion fields may possess an almost scale-invariant spectrum on large scales dependent upon the evolution of the dilaton and moduli fields to which they are coupled. As an example we calculate the spectra for three axion fields present in a truncated type IIB model and show that they are related with at least one of the fields having a scale-invariant or red perturbation spectrum. In the simplest pre big bang scenario this may be inconsistent with the observed isotropy of the microwave background. More generally, relations between the perturbation spectra in low energy string cosmologies reflect the symmetries of the theory.